Base station, user equipment, communication control method, and radio communication system

ABSTRACT

A base station includes a radio communication unit that establishes communication with a mobile communication terminal using a plurality of component carriers. The base station further includes a determination unit that determines a handover factor. The base station also includes a control unit allocates to the mobile communication terminal a measurement time interval for at least one component carrier from the plurality of component carriers according to the handover factor. A channel quality of the at least one component carrier of another base station is measured during the measurement time interval.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of, and claims thebenefit of priority under 35 U.S.C. §120 from, U.S. application Ser. No.13/503,037, filed Aug. 1, 2012, which claims the benefit of priorityunder 35 U.S.C. §119 from Japanese Patent Applications Nos. 2009-250476filed Oct. 30, 2009 and 2010-024409 filed on Feb. 5, 2010. The entirecontents of each of the above applications are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a base station, a user equipment, acommunication control method, and a radio communication system.

BACKGROUND ART

In Long Term Evolution-Advanced (LTE-A), which is the next-generationcellular communication standard that is discussed in Third GenerationPartnership Project (3GPP), introduction of technology called carrieraggregation (CA) has been studied. The carrier aggregation is technologythat forms a communication channel between a user equipment (UE) and abase station (BS, or evolved Node B (eNB)) by aggregating a plurality offrequency bands that are supported in LTE, for example, and therebyimproves communication throughput. Each frequency band included in onecommunication channel by the carrier aggregation is called a componentcarrier (CC). The bandwidths of frequency bands that are available inLTE are 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, and 20 MHz.Accordingly, if five bands of 20 MHz are aggregated as componentcarriers, a communication channel of 100 MHz in total can be formed.Component carriers that are included in one communication channel in thecarrier aggregation are not necessarily contiguous to one another in thefrequency direction. The mode in which component carriers are arrangedcontiguous to one another in the frequency direction is called acontiguous mode. On the other hand, the mode in which component carriersare arranged not contiguous to one another is called a non-contiguousmode.

Further, in the carrier aggregation, the number of component carriers inan uplink and the number of component carriers in a downlink are notnecessarily equal. The mode in which the number of component carriers inan uplink and the number of component carriers in a downlink are equalis called a symmetric mode. On the other hand, the mode in which thenumber of component carriers in an uplink and the number of componentcarriers in a downlink are not equal is called an asymmetric mode. Forexample, in the case of using two component carriers in an uplink andthree component carriers in a downlink, it is asymmetric carrieraggregation.

Further, in LTE, any one of frequency division duplex (FDD) and timedivision duplex (TDD) can be used as duplex operation. Because thedirection of a link (uplink or downlink) of each component carrier doesnot change in time in FDD, FDD is better suited to the carrieraggregation compared to TDD.

Meanwhile, a handover, which is a basic technique for achieving themobility of a user equipment in the cellular communication standard, isone of important subjects in LTE-A. In LTE, a user equipment measures acommunication quality over a channel with a serving base station (acurrently connected base station) and communication qualities withperipheral base stations and transmits a measurement report containingmeasurements to the serving base station. Receiving the measurementreport, the serving base station determines whether to execute ahandover based on the measurements contained in the report. Then, if itis determined that a handover is to be executed, a handover is carriedout among a source base station (the serving base station before ahandover), the user equipment, and a target base station (a serving basestation after a handover) in accordance with a prescribed procedure(e.g. cf. Patent Literature 1 below)

CITATION LIST Patent Literature

[PTL 1]

Japanese Unexamined Patent Application Publication No. 2009-232293

SUMMARY OF INVENTION Technical Problem

However, no case has been reported where active consideration is givento how to carry out a handover procedure in a radio communicationinvolving the carrier aggregation.

For example, in order to measure the communication quality with a basestation, it is generally required for a user equipment to establishsynchronization with a downlink channel from the base station. Afrequency to be synchronized is not necessarily the same as thefrequency being used for communication at that point of time. Therefore,the necessity arises in the user equipment to change the operationalfrequency of a radio communication unit in the physical layer. In orderto change the operational frequency, the base station allocates a periodcalled a measurement gap to the user equipment. The base station doesnot transmit data to the user equipment during the period of themeasurement gap, so that the user equipment is allowed to change theoperational frequency to perform measurement without any loss of data.However, in the case of involving the carrier aggregation, the number ofcomponent carriers that constitute one communication channel is plural.In this case, if measurement gaps are allocated to the respectivecomponent carriers in the same manner as it used to, the possibilityincreases that an increase in measurement gaps causes a decrease inthroughput or a delay in handover.

Further, the above issue related to an allocation of measurement gapscan occur not only at the time of a handover but also at the time of achange or an addition of a component carrier in a cell of one basestation in a radio communication involving the carrier aggregation.Assume, for example, that further improvement in throughput is desiredin the state of performing a radio communication involving the carrieraggregation between a user equipment and a base station. In such a case,throughput may be improved by measuring the communication quality of afrequency band not in use at the point of time and then changing theoperational frequency of a component carrier in use to the frequencyband with which suitable quality can be obtained or adding a componentcarrier whose operational frequency is that of the frequency band withwhich suitable quality can be obtained. In this case also, the necessityarises to allocate measurement gaps to the component carrier in use;however, there is a possibility that an allocation of measurement gapscauses a temporary decrease in throughput or a delay in processing.

In light of the foregoing, it is desirable to provide a novel andimproved base station, user equipment, communication control method, andradio communication system that can suppress a decrease in throughput ora delay in processing such as a handover due to an increase inmeasurement gaps in a radio communication involving the carrieraggregation.

Solution to Problem

According to some embodiments, a base station includes a radiocommunication unit configured to establish communication with a mobilecommunication terminal using a plurality of component carriers. The basestation further includes a determination unit configured to determine ahandover factor. The base station also includes a control unitconfigured to allocate to the mobile communication terminal ameasurement time interval for at least one component carrier from theplurality of component carriers according to the handover factor.Further, a channel quality of the at least one component carrier ofanother base station is measured during the measurement time interval.

According to some embodiments, a mobile communication terminal includesa radio communication unit configured to establish a communication witha base station using a plurality of component carriers. The mobilecommunication further includes a control unit configured to receive ameasurement time interval for at least one component carrier from a basestation according to a handover factor. The mobile communicationterminal also includes a measurement unit configured to measure achannel quality of the at least one component carrier of another basestation during the measurement time interval.

According to some embodiments, a method includes establishingcommunication with a mobile communication terminal using a plurality ofcomponent carriers. The method further includes determining a handoverfactor. The method also includes allocating to the mobile communicationterminal a measurement time interval for at least one component carrierfrom the plurality of component carriers according to the handoverfactor. Further, a channel quality of the at least one component carrierof another base station is measured during the measurement timeinterval.

According to some embodiments, a non-transitory computer readablemedium, having instructions stored thereon, which when executed by aprocessor in a base station causes the processor to establishcommunication with a mobile communication terminal using a plurality ofcomponent carriers. The instructions further cause the processor todetermine a handover factor. The instructions also cause the processorto allocate to the mobile communication terminal a measurement timeinterval for at least one component carrier from the plurality ofcomponent carriers according to the handover factor. Further, a channelquality of the at least one component carrier of another base station ismeasured during the measurement time interval.

According to some embodiments, a method includes establishing acommunication with a base station using a plurality of componentcarriers. The method further includes receiving a measurement timeinterval for at least one component carrier from a base stationaccording to a handover factor. The method also includes measuring achannel quality of the at least one component carrier of another basestation during the measurement time interval.

According to some embodiments, a non-transitory computer readablemedium, having instructions stored thereon, which when executed by aprocessor in a mobile communication terminal causes the processor toestablish a communication with a base station using a plurality ofcomponent carriers. The instructions further cause the processor toreceive a measurement time interval for at least one component carrierfrom a base station according to a handover factor. The instructionsalso cause the processor to measure a channel quality of the at leastone component carrier of another base station during the measurementtime interval.

According to some embodiments, a base station includes a radiocommunication unit configured to establish communication with a mobilecommunication terminal using a plurality of component carriers. The basestation further includes a determination unit configured to determinemoving speed of the mobile communication terminal or a channel qualityof component carrier. The base station also includes a control unitconfigured to allocate to the mobile communication terminal ameasurement time interval for at least one component carrier from theplurality of component carriers according to the moving speed of themobile communication terminal or the channel quality of componentcarrier. A channel quality of the at least one component carrier ismeasured during the measurement time interval.

Advantageous Effects of Invention

As described above, the base station, the user equipment, thecommunication control method, and the radio communication systemaccording to the embodiments of the present invention can suppress adecrease in throughput or a delay in processing such as a handover dueto an increase in measurement gaps in a radio communication involvingthe carrier aggregation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sequence chart to describe a flow of a typical handoverprocedure.

FIG. 2A is an explanatory view to describe an example of a structure ofa communication resource.

FIG. 2B is an explanatory view to describe measurement gaps.

FIG. 3 is a schematic view showing an outline of a radio communicationsystem according to an embodiment.

FIG. 4 is an explanatory view to describe an issue related to ameasurement gap at the time of carrier aggregation.

FIG. 5 is a block diagram showing an example of a configuration of auser equipment according to an embodiment.

FIG. 6 is a block diagram showing an example of a detailed configurationof a radio communication unit according to an embodiment.

FIG. 7 is a block diagram showing an example of a configuration of abase station according to an embodiment.

FIG. 8 is a block diagram showing an example of a detailed configurationof a determination unit according to an embodiment.

FIG. 9 is a flowchart showing an example of a flow of an urgency leveldetermination process according to an embodiment.

FIG. 10 is a flowchart showing an example of a detailed flow of ameasurement gap allocation process according to an embodiment.

FIG. 11A is an explanatory view to describe a first example of anallocation of measurement gaps.

FIG. 11B is an explanatory view to describe a second example of anallocation of measurement gaps.

FIG. 11C is an explanatory view to describe a third example of anallocation of measurement gaps.

FIG. 12 is a flowchart showing an example of a detailed flow of ameasurement gap allocation process according to an alternative example.

FIG. 13 is an explanatory view to describe a change or an addition of acomponent carrier.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the appended drawings. Note that,in this specification and the appended drawings, structural elementsthat have substantially the same function and structure are denoted withthe same reference numerals, and repeated explanation of thesestructural elements is omitted.

Preferred embodiments of the present invention will be describedhereinafter in the following order.

1. Description of Related Art

1-1. Handover Procedure

1-2. Measurement Gap

2. Outline of Radio Communication System

2-1. Overview of System

2-2. Issue Related to Carrier Aggregation

3. Exemplary Configuration of Device According to Embodiment

3-1. Exemplary Configuration of User Equipment

3-2. Exemplary Configuration of Base Station

4. Example of Process According to Embodiment

4-1. Flow of Process

4-2. Example of Allocation of Measurement Gaps

4-3. Alternative Example

5. Example of Application to Change or Addition of Component Carrier

6. Summary

1. Description of Related Art

(1-1. Handover Procedure)

A technique related to the present invention is described hereinafterwith reference to FIGS. 1, 2A and 2B. FIG. 1 shows a flow of a handoverprocedure in conformity with LTE in a radio communication not involvingthe carrier aggregation as an example of a typical handover procedure.In this example, a user equipment (UE), a source base station (sourceeNB), a target base station (target eNB), and a mobility managemententity (MME) are involved in the handover procedure.

As a preliminary step toward a handover, the user equipment firstreports the channel quality of a communication channel between the userequipment and the source base station to the source base station (stepS2). The channel quality may be reported on a regular basis or when thechannel quality falls below a predetermined reference value. The userequipment can measure the channel quality of the communication channelwith the source base station by receiving a reference signal containedin a downlink channel from the source base station.

Then, the source base station determines the needs of measurement basedon the quality report received from the user equipment and, ifmeasurement is necessary, allocates measurement gaps to the userequipment (step S4). The measurement gap is described in further detaillater.

Then, the user equipment searches for a downlink channel from aperipheral base station (i.e. performs cell search) during the periodsof the allocated measurement gaps (step S12). Note that the userequipment can recognize a peripheral base station to search according toa list that is provided in advance from the source base station.

When the user equipment acquires synchronization with a downlinkchannel, the user equipment performs measurement by using a referencesignal contained in the downlink channel (step S14). During this period,the source base station restricts an allocation of data communicationrelated to the user equipment so as to avoid occurrence of datatransmission by the user equipment.

Upon completion of the measurement, the user equipment transmits ameasurement report containing measurements to the source base station(step S22). The measurements contained in the measurement report may bethe average value or the central value of measured values over aplurality of times of measurement or the like. Further, the measurementsmay contain data about a plurality of frequency bands.

Receiving the measurement report, the source base station determineswhether or not to execute a handover based on the contents of themeasurement report. For example, when the channel quality of anotherbase station in the periphery is higher than the channel quality of thesource base station by a predetermined threshold or greater, it can bedetermined that a handover is necessary. In this case, the source basestation determines to carry out a handover procedure with the relevantanother base station as a target base station, and transmits a handoverrequest to the target base station (step S24).

Receiving the handover request, the target base station determineswhether it is possible to accept the user equipment according to theavailability of a communication service offered by itself or the like.When it is possible to accept the user equipment, the target basestation transmits a handover request confirm to the source base station(step S26).

Receiving the handover request confirm, the source base stationtransmits a handover command to the user equipment (step S28). Then, theuser equipment acquires synchronization with the downlink channel of thetarget base station (S32). After that, the user equipment makes randomaccess to the target base station by using a random access channel in agiven time slot (step S34). During this period, the source base stationforwards data addressed to the user equipment to the target base station(step S36). Then, after succeeded in the random access, the userequipment transmits a handover complete to the target base station (stepS42).

Receiving the handover complete, the target base station requests theMME to perform route update for the user equipment (step S44). Uponupdating the route of user data by the MME, the user equipment becomesable to communicate with another device through a new base station (i.e.the target base station). Then, the target base station transmitsacknowledgement to the user equipment (step S46). A series of handoverprocedure thereby ends.

(1-2. Measurement Gap)

FIG. 2A shows a structure of a communication resource in LTE as anexample of a structure of a communication resource to which the presentinvention is applicable. Referring to FIG. 2A, the communicationresource in LTE is segmented in the time direction into radio frameseach having a length of 10 msec. One radio frame includes tensub-frames, and one sub-frame is made up of two 0.5 msec slots. In LTE,the sub-frame is one unit of an allocation of a communication resourceto each user equipment in the time direction. Such one unit is called aresource block. One resource block includes twelve sub-carriers in thefrequency direction. Specifically, one resource block has a size of 1msec with 12 sub-carriers in the time-frequency domain. Throughput ofdata communication increases as a larger number of resource blocks areallocated for data communication on condition of the same bandwidth andtime length.

FIG. 2B is an explanatory view to describe typical measurement gaps.Referring to FIG. 2B, a measurement gap MG1 is allocated at the positioncorresponding to the second radio frame from the left in the timedirection. Further, a measurement gap MG2 is allocated at the positioncorresponding to the fourth radio frame from the left in the timedirection. Each measurement gap generally has a length of 6 msec. Theuser equipment can use 5.166 msec at the center for measurement (cf.MG1a in FIG. 2B). The first part of the remaining part of themeasurement gap is used for tuning of the operational frequency to thefrequency band as a target of measurement by the user equipment (cf.MG1b in FIG. 2B). Further, the latter part of the remaining part of themeasurement gap is used for re-tuning of the operational frequency fromthe frequency band as a target of measurement to the original frequencyband by the user equipment (cf. MG1c in FIG. 2B). The interval of themeasurement gaps is generally set to be the integral multiple of a radioframe length. Note that, when the frequency band as a target ofmeasurement is the same as the original operational frequency, it is notnecessary to allocate measurement gaps. In this case, the user equipmentcan perform measurement without changing its operational frequency inthe physical layer. Such measurement gaps can be allocated to the userequipment in the preliminary step toward a handover procedure as shownin FIG. 1, and they can be further allocated to the user equipment atthe time of changing or adding a component carrier.

2. Outline of Radio Communication System

(2-1. Overview of System)

An outline of a radio communication system to which the presentinvention is applicable is described hereinafter with reference to FIGS.3 and 4.

FIG. 3 is a schematic view showing an outline of a radio communicationsystem 1 according to an embodiment of the present invention. Referringto FIG. 3, the radio communication system 1 includes a user equipment100, a base station 200 a and a base station 200 b. It is assumed thatthe base station 200 a is a serving base station for the user equipment100.

The user equipment 100 is located inside a cell 202 a where a radiocommunication service is provided by the base station 200 a. The userequipment 100 can perform a data communication with another userequipment (not shown) via the base station 200 a over a communicationchannel formed by aggregating a plurality of component carriers (i.e. bycarrier aggregation). However, because the distance between the userequipment 100 and the base station 200 a is not short, there is apossibility that a handover is required for the user equipment 100.Further, the user equipment 100 is located inside a cell 202 b where aradio communication service is provided by the base station 200 b.Therefore, the base station 200 b can be a candidate for a target basestation for a handover of the user equipment 100.

The base station 200 a can communicate with the base station 200 bthrough a backhaul link (e.g. X2 interface). Various kinds of messagesin the handover procedure as described with reference to FIG. 1,scheduling information related to the user equipment belonging to eachcell or the like, for example, can be transmitted and received betweenthe base station 200 a and the base station 200 b. Further, the basestation 200 a and the base station 200 b can communicate with the MME,which is an upper node, through S1 interface, for example.

It should be noted that, when there is no particular need to distinguishbetween the base station 200 a and the base station 200 b in thefollowing description of the specification, they are collectivelyreferred to as a base station 200 by omitting the alphabetical letter atthe end of the reference symbol. The same applies to the other elements.

(2-2. Issue Related to Carrier Aggregation)

In the case where the user equipment 100 may perform a carrieraggregation under the circumstance that there is a possibility of ahandover as shown in FIG. 3, the issue arises as to how to allocatemeasurement gaps to a plurality of component carriers constituting acommunication channel. FIG. 4 is an explanatory view to describe such anissue related to measurement gaps at the time of the carrieraggregation.

Generally, there are a plurality of candidates for a frequency bandafter a handover which is to be a target of measurement even in the caseof not performing a carrier aggregation. In the case of involving thecarrier aggregation, the necessary number of times of measurementincreases according to the number of component carriers. In the exampleof FIG. 4, three component carriers CC1 to CC3 that are partlydiscretely arranged in the frequency direction constitute acommunication channel between the user equipment and the serving basestation. Further, the number of candidates for a component carrier aftera handover to be a target of measurement involving a change inoperational frequency is three for each of the component carriers. Inthe example of FIG. 4, even when performing measurement once foroverlapping candidates, it is necessary to carry out measurement forseven component carriers in the target base station for the overallcommunication channel. Note that, if the number of component carriers inuse is three and the number of candidates for a component carrier aftera handover for each of the component carriers is three, 3×3=9 times ofmeasurement at the maximum are necessary when calculated in the mostsimplified manner.

An increase in the necessary number of times of measurement means thatit is necessary to allocate a larger number of measurement gaps forchanging the operational frequency at the time of measurement (which caninclude tuning and re-tuning). This leads to a decrease in throughputand a delay in handover associated with the stop of data communicationin the middle of a measurement gap. Therefore, in the radiocommunication system 1 where the carrier aggregation is performed, it iseffective to perform an allocation of measurement gaps efficiently by atechnique according to an embodiment described in the followingsections.

3. Exemplary Configuration of Device According to Embodiment

Examples of configurations of the user equipment 100 and the basestation 200 included in the radio communication system 1 according to anembodiment of the present invention are described hereinafter withreference to FIGS. 5 to 8.

(3-1. Exemplary Configuration of User Equipment)

FIG. 5 is a block diagram showing an example of a configuration of theuser equipment 100 according to the embodiment. Referring to FIG. 5, theuser equipment 100 includes a radio communication unit 110, a signalprocessing unit 150, a control unit 160, and a measurement unit 170.

(Radio Communication Unit)

The radio communication unit 110 performs a radio communication with thebase station 200 over a communication channel formed by aggregating aplurality of component carriers with use of the carrier aggregationtechnology.

FIG. 6 is a block diagram showing an example of a more detailedconfiguration of the radio communication unit 110. Referring to FIG. 6,the radio communication unit 110 includes an antenna 112, a switch 114,a low noise amplifier (LNA) 120, a plurality of down-converters 122 a to122 c, a plurality of filters 124 a to 124 c, a plurality ofanalogue-to-digital converters (ADCs) 126 a to 126 c, a demodulationunit 128, a modulation unit 130, a plurality of digital-to-analogueconverters (DACs) 132 a to 132 c, a plurality of filters 134 a to 134 c,a plurality of up-converters 136 a to 136 c, a combiner 138, and a poweramplifier (PA) 140.

The antenna 112 receives a radio signal transmitted from the basestation 200 and outputs the received signal to the LNA 120 through theswitch 114. The LNA 120 amplifies the received signal. Thedown-converter 122 a and the filter 124 a separate a baseband signal ofthe first component carrier (CC1) from the received signal amplified bythe LNA 120. Then, the separated baseband signal is converted to adigital signal by the ADC 126 a and output to the demodulation unit 128.Likewise, the down-converter 122 b and the filter 124 b separate abaseband signal of the second component carrier (CC2) from the receivedsignal amplified by the LNA 120. Then, the separated baseband signal isconverted to a digital signal by the ADC 126 b and output to thedemodulation unit 128. Further, the down-converter 122 c and the filter124 c separate a baseband signal of the third component carrier (CC3)from the received signal amplified by the LNA 120. Then, the separatedbaseband signal is converted to a digital signal by the ADC 126 c andoutput to the demodulation unit 128. After that, the demodulation unit128 generates a data signal by demodulating the baseband signals of therespective component carriers and outputs the data signal to the signalprocessing unit 150.

Further, when a data signal is input from the signal processing unit150, the modulation unit 130 modulates the data signal and generatesbaseband signals of the respective component carriers. Among thosebaseband signals, the baseband signal of the first component carrier(CC1) is converted to an analog signal by the DAC 132 a. Then, afrequency component corresponding to the first component carrier in atransmission signal is generated from the analog signal by the filter134 a and the up-converter 136 a. Likewise, the baseband signal of thesecond component carrier (CC2) is converted to an analog signal by theDAC 132 b. Then, a frequency component corresponding to the secondcomponent carrier in the transmission signal is generated from theanalog signal by the filter 134 b and the up-converter 136 b. Further,the baseband signal of the third component carrier (CC3) is converted toan analog signal by the DAC 132 c. Then, a frequency componentcorresponding to the third component carrier in the transmission signalis generated from the analog signal by the filter 134 c and theup-converter 136 c. After that, the generated frequency componentscorresponding to the three component carriers are combined by thecombiner 138, and the transmission signal is formed. The PA 140amplifiers the transmission signal and outputs the transmission signalto the antenna 112 through the switch 114. Then, the antenna 112transmits the transmission signal as a radio signal to the base station200.

Although the case where the radio communication unit 110 handles threecomponent carriers is described in FIG. 6, the number of componentcarriers handled by the radio communication unit 110 may be two, or fouror more.

Further, instead of processing the signals of the respective componentcarriers in the analog region as in the example of FIG. 6, the radiocommunication unit 110 may process the signals of the respectivecomponent carriers in the digital region. In the latter case, at thetime of reception, a digital signal converted by one ADC is separatedinto the signals of the respective component carriers by a digitalfilter. Further, at the time of transmission, after digital signals ofthe respective component carriers are frequency-converted and combined,the signal is converted into an analog signal by one DAC. The load ofthe ADC and the DAC is generally smaller when processing the signals ofthe respective component carriers in the analog region. On the otherhand, when processing the signals of the respective component carriersin the digital region, a sampling frequency for AD/DA conversion ishigher, and the load of the ADC and the DAC can thereby increase.

(Signal Processing Unit)

Referring back to FIG. 5, an example of a configuration of the userequipment 100 is further described below.

The signal processing unit 150 performs signal processing such asdeinterleaving, decoding or error correction on the demodulated datasignal that is input from the radio communication unit 110. Then, thesignal processing unit 150 outputs the processed data signal to an upperlayer. Further, the signal processing unit 150 performs signalprocessing such as encoding or interleaving on the data signal that isinput from the upper layer. Then, the signal processing unit 150 outputsthe processed data signal to the radio communication unit 110.

(Control Unit)

The control unit 160 controls the overall functions of the userequipment 100 by using a processing device such as a central processingunit (CPU) or a digital signal processor (DSP). For example, the controlunit 160 controls the timing of data communication by the radiocommunication unit 110 according to scheduling information that isreceived from the base station 200 by the radio communication unit 110.Further, the control unit 160 controls the measurement unit 170 tomeasure the channel quality by using a reference signal from the basestation 200, which is a serving base station, and transmits the channelquality report to the base station 200 through the radio communicationunit 110. Further, in this embodiment, when measurement gaps areallocated to the user equipment 100 by the base station 200, the controlunit 160 controls the measurement unit 170 to execute measurement duringthe periods of the allocated measurement gaps.

(Measurement Unit)

The measurement unit 170 measures the channel quality for each of thecomponent carriers by using a reference signal from the base station 200according to control from the control unit 160, for example. Further,when measurement gaps are allocated to the user equipment 100 by thebase station 200, the measurement unit 170 executes measurement for ahandover by using the allocated measurement gaps.

In this embodiment, measurement gaps are allocated for each of thecomponent carriers by the base station 200 as described later. In lightof this, during the period of the measurement gap allocated to the firstcomponent carrier, for example, the control unit 160 tunes theoperational frequency for the first component carrier (e.g. theoperational frequency of the branch of the down-converter 122 a, thefilter 124 a and the ADC 126 a shown in FIG. 6) of the radiocommunication unit 110 to a given frequency band as a target ofmeasurement. Next, the measurement unit 170 performs measurement for therelevant frequency band. Then, before the period of the measurement gapends, the control unit 160 re-tunes the operational frequency for thefirst component carrier of the radio communication unit 110 to theoriginal frequency band. Such measurement is performed in the samemanner for the second and third component carriers also.

The measurements for each of the component carriers executed by themeasurement unit 170 in the above manner are converted to apredetermined format for a measurement report by the control unit 160and transmitted to the base station 200 through the radio communicationunit 110. After that, the base station 200 determines, based on themeasurement report, whether to execute a handover for the user equipment100.

(3-2. Exemplary Configuration of Base Station)

FIG. 7 is a block diagram showing an example of a configuration of thebase station 200 according to the embodiment. Referring to FIG. 7, thebase station 200 includes a radio communication unit 210, an interfaceunit 250, a component carrier (CC) management unit 260, a determinationunit 270, and a control unit 280.

(Radio Communication Unit)

A specific configuration of the radio communication unit 210 may besimilar to the configuration of the radio communication unit 110 of theuser equipment 100 which is described above with reference to FIG. 6,although the number of component carriers to be supported, therequirements of processing performance or the like are different. Theradio communication unit 210 performs a radio communication with theuser equipment over a communication channel formed by aggregating aplurality of component carriers with use of the carrier aggregationtechnology.

(Interface Unit)

The interface unit 250 mediates a communication between the radiocommunication unit 210 or the control unit 280 and an upper node throughthe S1 interface illustrated in FIG. 3, for example. Further, theinterface unit 250 mediates a communication between the radiocommunication unit 210 or the control unit 280 and another base stationthrough the X2 interface illustrated in FIG. 3, for example.

(CC Management Unit)

The CC management unit 260 holds data that indicates which componentcarrier each user equipment is using for communication with respect toeach of the user equipments belonging to the cell of the base station200. Such data can be updated by the control unit 280 when an additionaluser equipment joins the cell of the base station 200 or when theexisting user equipment changes its component carriers. Thus, thedetermination unit 270 and the control unit 280, for example, canrecognize which component carrier the user equipment 100 is using byreferring to the data held by the CC management unit 260.

(Determination Unit)

The determination unit 270 determines the urgency level of a handover ofthe user equipment based on a received signal that is received from theuser equipment by the radio communication unit 210. Specifically, thedetermination unit 270 may determine that the urgency level of ahandover of the user equipment is higher as the moving speed of the userequipment which is detected based on the received signal received fromthe user equipment is higher. Further, the determination unit 270 maydetermine that the urgency level of a handover of the user equipment ishigher as the channel quality contained in the received signal receivedfrom the user equipment is lower.

FIG. 8 is a block diagram showing an example of a more detailedconfiguration of the determination unit 270. Referring to FIG. 8, thedetermination unit 270 includes a quality acquisition unit 272, a speeddetection unit 274, and an urgency level determination unit 276.

The quality acquisition unit 272 acquires the quality level of acommunication channel between the user equipment and the base station200 for each of the component carriers based on the received signal fromthe user equipment. For example, the quality acquisition unit 272 mayacquire the quality levels of the respective component carriers byreceiving the channel quality report transmitted from the userequipment. Further, the quality acquisition unit 272 may acquire thequality levels of the respective component carriers by measuring aparameter such as a received signal intensity or an error rate of thereceived signal from the user equipment. The quality acquisition unit272 outputs the quality levels of the respective component carriersacquired in this manner to the urgency level determination unit 276.

The speed detection unit 274 detects the moving speed of the userequipment based on the received signal received from the user equipment.For example, in the case where the user equipment has a globalpositioning system (GPS) function, position data indicating the positionof the user equipment measured by the GPS function is contained in thereceived signal. In this case, the speed detection unit 274 can detectthe moving speed of the user equipment by acquiring the position datafrom the received signal and then calculating a change over time of theposition of the user equipment by using the acquired position data.Further, the moving speed may be calculated in the user equipment fromthe position measured by the GPS function. In this case, the speeddetection unit 274 can be informed of the moving speed from the userequipment. Further, the speed detection unit 274 may detect the movingspeed of the user equipment from a measurement result about the receivedsignal from the user equipment, such as a change in the signal delay ofthe received signal, for example. Further, the speed detection unit 274may measure the position of the user equipment by using knownpositioning technology based on a radio signal and calculate the movingspeed of the user equipment. The speed detection unit 274 outputs themoving speed of the user equipment detected in this manner to theurgency level determination unit 276.

The urgency level determination unit 276 determines the urgency level ofa handover of the user equipment based on the channel quality levelinput from the quality acquisition unit 272 and the moving speed of theuser equipment input from the speed detection unit 274. FIG. 9 is aflowchart showing an example of a flow of an urgency level determinationprocess by the determination unit 270 according to the embodiment.

According to some embodiments, a handover factor specifies an urgencylevel for handover to the another base station. According to furtherembodiments the urgency level depends on a moving speed of the mobilecommunication terminal or a communication quality of the mobilecommunication terminal. Further, according to some embodiments, theurgency level is low upon determination that the moving speed of themobile communication terminal is below a speed threshold and thecommunication quality is above a communication threshold.

Referring to FIG. 9, the quality acquisition unit 272 first acquires thequality levels of the respective component carriers (step S102). Next,the speed detection unit 274 detects the moving speed of the userequipment (step S104).

Then, the urgency level determination unit 276 determines whether thechannel quality is lower than a predetermined reference by using thequality levels of the respective component carriers (step S106).Specifically, for example, the urgency level determination unit 276compares a parameter such as the average value or the minimum value ofthe quality levels of the respective component carriers with apredetermined reference value such as a communication threshold. When itis determined that the channel quality is lower than the predeterminedreference, the process proceeds to the step S112. On the other hand,when it is determined that the channel quality is not lower than thepredetermined reference, the process proceeds to the step S108.

In the step S108, the urgency level determination unit 276 determineswhether the moving speed of the user equipment is higher than apredetermined reference value such as a speed threshold (step S108).When it is determined that the moving speed of the user equipment ishigher than the predetermined reference, the process proceeds to thestep S112. On the other hand, when it is determined that the movingspeed of the user equipment is not higher than the predeterminedreference, the process proceeds to the step S110.

In the step S110, because the channel quality is not lower than thepredetermined reference and the moving speed of the user equipment isnot higher than the predetermined reference, the urgency leveldetermination unit 276 determines that the urgency level of a handoverof the user equipment is low (step S110). On the other hand, in the stepS112, because the channel quality is lower than the predeterminedreference and the moving speed of the user equipment is higher than thepredetermined reference, the urgency level determination unit 276determines that the urgency level of a handover of the user equipment ishigh (step S112).

The urgency level determination unit 276 outputs the urgency leveldetermined in this manner to the control unit 280. Further, the qualityacquisition unit 272 outputs the quality levels of the respectivecomponent carriers to the control unit 280. Note that the case ofdetermining whether the urgency level of a handover of the userequipment is either “high” or “low” is described in FIG. 9. However, thepresent invention is not limited thereto, and the urgency level of ahandover may be categorized into a larger number of levels. Further, thereference value to be compared with a parameter such as the averagevalue or the minimum value of the quality levels of the respectivecomponent carriers in the step S106 of FIG. 9 may vary dynamically, forexample. The reference value may vary dynamically based on the number ofuser equipments connected to the base station, the surrounding electricfield environment or the like, for example. This enables a flexiblesystem operation. Likewise, the reference value to be compared with themoving speed of the user equipment in the step S108 may also varydynamically.

(Control Unit)

The control unit 280 controls the overall functions of the base station200 by using a processing device such as a CPU or a DSP. Further, inthis embodiment, the control unit 280 controls an allocation ofmeasurement gaps for the user equipment with respect to each componentcarrier according to the urgency level determined as a result of theurgency level determination process by the determination unit 270described above.

Specifically, the control unit 280 can allocate measurement gaps to alarger number of component carriers as the urgency level of a handoveris higher. For example, the case is assumed where the urgency level of ahandover is categorized to either “high” or “low” as described withreference to FIG. 9. In this case, when the urgency level of a handoveris determined to be high, the control unit 280 allocates measurementgaps to all of the component carriers being used by the user equipment.On the other hand, when the urgency level of a handover is determined tobe low, the control unit 280 allocates measurement gaps to some (e.g.any one) of the component carriers being used by the user equipment.Thus, when the urgency level of a handover is low, measurement isperformed over a relatively long time to thereby avoid a decrease inthroughput, and when the urgency level of a handover is high,measurement is performed in a short time to thereby prevent a delay inhandover.

Further, the control unit 280 may vary the pattern of an allocation ofmeasurement gaps depending on the quality levels of the channel qualityof the respective component carriers acquired by the quality acquisitionunit 272.

Specifically, in the case of allocating measurement gaps to some of thecomponent carriers, for example, the control unit 280 may preferentiallyallocate measurement gaps to a component carrier with a low qualitylevel. Further, in the case of allocating measurement gaps to two ormore component carriers, the control unit 280 may set the interval ofmeasurement gaps allocated to a first component carrier to be longerthan the interval of measurement gaps allocated to a second componentcarrier with a lower quality level. The proportion of a measurement gapto the communication resource is thereby low for the component carrierwith a high quality level, and it is thereby possible to suppress adecrease in throughput compared to the case of allocating measurementgaps uniformly to all component carriers regardless of the qualitylevel.

Further, in the case of allocating measurement gaps to two or morecomponent carriers, for example, the control unit 280 may decide anallocation of measurement gaps in such a way that the timing of anymeasurement gap does not coincide with the timing of another measurementgap. It is thereby possible to avoid a delay in data transmission due tothe existence of a time during which data transmission is not performedat all.

Further, in the case where two or more component carriers are contiguousto one another in the frequency direction or where the distance betweentwo or more component carriers in the frequency direction is shorterthan a predetermined threshold, for example, the control unit 280 mayallocate measurement gaps only to one component carrier of the two ormore component carriers. In such a case, the measurements for onecomponent carrier are used for the other component carrier located incontiguous or close proximity in the frequency direction, for example,so that it is possible to shorten a time necessary for measurement andthereby avoid a delay in handover and suppress a decrease in throughput.

Further, in the case where the number of component carriers availablefor measurement is notified from a user equipment, for example, thecontrol unit 280 may control an allocation of measurement gaps in such away that the number of component carriers to which measurement gaps areallocated does not exceed the notified number. It is thereby possible toavoid a useless allocation of measurement gaps and suppress a decreasein throughput.

4. Example of Process According to Embodiment

(4-1. Flow of Process)

FIG. 10 is a flowchart showing an example of a detailed flow of ameasurement gap allocation process by the base station 200 according toan embodiment.

Referring to FIG. 10, the radio communication unit 210 first receivesthe channel quality report from the user equipment (step S202). Then,the radio communication unit 210 outputs the received channel qualityreport to the control unit 280.

Next, the control unit 280 determines the necessity of measurement for ahandover based on the channel quality report (step S204). When it isdetermined that measurement for a handover is unnecessary for the reasonsuch as suitable channel quality, for example, measurement gaps are notallocated (step S206), and the measurement gap allocation process ends.On the other hand, when it is determined that measurement for a handoveris necessary, the process proceeds to the step S208.

In the step S208, the urgency level determination process describedabove with reference to FIG. 9 is performed by the determination unit270 (step S208). Then, the determination unit 270 outputs the determinedurgency level of a handover to the control unit 280.

Then, the control unit 280 determines whether the urgency level of ahandover determined by the determination unit 270 is high or not (stepS210). When the urgency level of a handover is high, the processproceeds to the step S212. On the other hand, when the urgency level ofa handover is not high, the process proceeds to the step S214.

In the step S212, the control unit 280 allocates measurement gaps to allof the component carriers being used by the user equipment (step S212).On the other hand, in the step S214, the control unit 280 allocatesmeasurement gaps to some of the component carriers being used by theuser equipment (step S214). Then, the measurement gap allocation processends.

(4-2. Example of Allocation of Measurement Gaps)

According to some embodiments, when a component carrier from theplurality of component carriers has a higher communication qualitycompared to each other component carrier from the plurality of componentcarriers, assignment of the measurement gap to the component carrierwith the highest communication quality is avoided. In furtherembodiments a frequency of allocation of the measurement gap is adjustedwhere the frequency of the measurement time interval is increased if thecommunication quality of a component carrier degrades, and the frequencyof the measurement time interval is decreased if the communicationquality of the component carrier improves.

FIGS. 11A to 11C respectively show examples of the pattern ofmeasurement gaps allocated as a result of the measurement gap allocationprocess by the control unit 280.

(Pattern A)

Referring to FIG. 11A, one communication channel is made up of threecomponent carriers CC1 to CC3. The component carriers CC1 to CC3 are notlocated in close proximity to one another in the frequency direction.Further, it is assumed that the urgency level of a handover isdetermined to be high by the determination unit 270.

In this case, the control unit 280 allocates measurement gaps to all ofthe component carriers CC1 to CC3. In the example of FIG. 11A,measurement gaps MG11, MG12, . . . are allocated to the componentcarrier CC1. Further, measurement gaps MG13, MG14, . . . are allocatedto the component carrier CC2. Further, measurement gaps MG15, . . . areallocated to the component carrier CC3. Thus, the user equipmentperforms measurement in a short time, thereby enabling a handover to beexecuted promptly.

(Pattern B)

Referring to FIG. 11B, one communication channel is made up of threecomponent carriers CC1 to CC3 as in FIG. 11A. The component carriers CC1and CC2 are located in close proximity to each another in the frequencydirection. Further, it is assumed that the urgency level of a handoveris determined to be high by the determination unit 270. Further, thequality level of the component carrier CC1 is higher than the qualitylevel of the component carrier CC2.

In this case, the control unit 280 allocates measurement gaps to thecomponent carrier CC2 with the lower quality level, for example, of thecomponent carriers CC1 and CC2 which are located in close proximity toeach another in the frequency direction. Further, the control unit 280allocates measurement gaps also to the component carrier CC3. In theexample of FIG. 11B, measurement gaps MG21, MG22, . . . are allocated tothe component carrier CC2. Further, measurement gaps MG23, MG24 . . .are allocated to the component carrier CC3. On the other hand, nomeasurement gap is allocated to the component carrier CC1 that largelycontributes to throughput (i.e. with a high quality level). Thus, in thepattern B, a decrease in throughput due to an allocation of measurementgaps is effectively suppressed.

(Pattern C)

Referring to FIG. 11C, one communication channel is made up of threecomponent carriers CC1 to CC3 as in FIGS. 11A and 11B. Further, it isassumed that the urgency level of a handover is determined to be low bythe determination unit 270. Further, the quality levels of the componentcarriers CC1, CC2 and CC3 are low, intermediate and high, respectively.

In this case, the control unit 280 allocates more communicationresources for measurement gaps as the quality level of each componentcarrier is lower. In the example of FIG. 11C, no measurement gap isallocated to the component carrier CC3 with the highest quality level.On the other hand, measurement gaps MG31, MG32, MG33 . . . are allocatedto the component carrier CC1. Further, measurement gaps MG34, MG35 . . .are allocated to the component carrier CC2. However, the interval T1 ofthe measurement gaps in the component carrier CC1 is two radio frames,and the interval T2 of the measurement gaps in the component carrier CC2is four radio frames. In this manner, the interval of the measurementgaps of the component carrier CC2 with a higher quality level is setlonger, so that a decrease in throughput in the communication channel asa whole is effectively suppressed.

Further, in any of the examples of FIGS. 11A to 11C, an allocation ofmeasurement gaps is decided so that the timing of any measurement gapdoes not coincide with the timing of another measurement gap. Thiseliminates the existence of a time during which data transmission is notperformed at all, so that a delay in data transmission is avoided.

4-3. Alternative Example

In the above embodiment, the case where the base station determines theurgency level of a handover of the user equipment and controls anallocation of measurement gaps according to the urgency level and thequality levels of the respective component carriers is described by wayof illustration. However, the base station may control an allocation ofmeasurement gaps according to the quality levels of the respectivecomponent carriers without determining the urgency level of a handoverof the user equipment. FIG. 12 shows an example of a flow of ameasurement gap allocation process according to such an alternativeexample of the embodiment.

Referring to FIG. 12, a radio communication unit 210 first receives thechannel quality report from the user equipment (step S302). Then, theradio communication unit 210 outputs the received channel quality reportto the control unit 280.

Next, the control unit 280 determines the necessity of measurement for ahandover based on the channel quality report (step S304). When it isdetermined that measurement for a handover is unnecessary for the reasonsuch as suitable channel quality, for example, measurement gaps are notallocated (step S306), and the measurement gap allocation process ends.On the other hand, when it is determined that measurement for a handoveris necessary, the process proceeds to the step S308.

In the step S308, the control unit 280 allocates measurement gaps foreach of the component carriers according to the quality levels of therespective component carriers acquired by the quality acquisition unit272 (step S308). In this step, as described above with reference toFIGS. 11B and 11C, for example, measurement gaps can be preferentiallyallocated to a component carrier with a low quality level. Further,measurement gaps with a shorter interval can be allocated to a componentcarrier with a low quality level. The measurement gap allocation processthereby ends.

Note that, in the above measurement gap allocation process or themeasurement gap allocation process described earlier with reference toFIG. 10, a constraint on the number of component carriers to whichmeasurement gaps are allocable may be set in advance, for example. Forexample, the number of component carriers to which measurement gaps areallocable may be always one, and measurement gaps may be allocated to acomponent carrier with the lowest quality level. Further, when one or aplurality of component carriers (or RF circuits) where measurement isexecutable is specified in advance, measurement gaps may be allocated tothe specified one or plurality of component carriers (or RF circuits).

5. Example of Application to Change or Addition of Component Carrier

The above-described technique related to control of an allocation ofmeasurement gaps is also applicable to a change of a component carrier(a change of the operational frequency of a component carrier) or anaddition of a component carrier in the user equipment 100 in the cell ofone base station 200.

According to some embodiments, a component carrier used forcommunication with the mobile terminal is deleted or added from aplurality of component carriers according to at least one of acommunication quality and a measurement report. FIG. 13 is anexplanatory view to describe an example of applying the above-describedembodiment to a change or an addition of a component carrier. Note thatit is assumed in the scenario of FIG. 13 that the user equipment 100performs a radio communication involving the carrier aggregation withthe base station 200 acting as a serving base station. A sequence chartabout a procedure of changing a component carrier between the userequipment 100 and the base station 200 is shown on the right of FIG. 13.The state of the operational frequency in each stage of the sequence isshown on the left of FIG. 13.

Referring to FIG. 13, the user equipment 100 first performs a radiocommunication with the base station 200 by using three componentcarriers CC1 to CC3. The frequency bands in operation of the componentcarriers CC1, CC2 and CC3 are the first band (#1), the second band (#2)and the third band (#3), respectively,

The user equipment 100 first reports the channel qualities of therespective component carriers to the base station 200 (step S402). Thechannel quality may be reported on a regular basis or when the channelquality falls below a predetermined reference value. Further, instead ofthe channel quality report, the user equipment 100 may transmit acomponent carrier change (or addition) request for a throughput increaseto the base station 200.

Next, the base station 200 allocates measurement gaps to the userequipment 100 by the measurement gap allocation process which isdescribed earlier with reference to FIG. 10 or 12, for example (stepS404).

Specifically, measurement gaps can be preferentially allocated to acomponent carrier with a low quality level among the component carriersCC1, CC2 and CC3, for example. Further, measurement gaps with a shorterinterval can be allocated to a component carrier with a low qualitylevel.

Then, during the periods of the allocated measurement gaps, the userequipment 100 acquires synchronization with a downlink channel from thebase station 200 for a frequency band not in use and performsmeasurement by using a reference signal contained in the downlinkchannel (step S412). In the example of FIG. 13, measurement for thefourth band (#4) is performed during the period of the measurement gapallocated to the component carrier CC2. Further, measurement for thefifth band (#5) is performed during the period of the measurement gapallocated to the component carrier CC3.

After measurement, the user equipment 100 transmits a measurement reportcontaining measurements to the base station 200 (step S414). Themeasurements contained in the measurement report may be the averagevalue or the central value of measured values over a plurality of timesof measurement or the like.

Receiving the measurement report, the base station 200 determines thenecessity of a change or an addition of a component carrier of the userequipment 100 based on the contents of the measurement report. Forexample, in the case where a frequency band having a higher quality thanthe channel quality of any of the component carriers CC1 to CC3 exists,it can be decided that the operational frequency of the componentcarrier should be changed to the frequency band having the higherquality. Further, in the case where the number of component carrierscurrently used in the user equipment 100 is smaller than the number ofavailable component carriers and another frequency band having a highquality exists, it can be decided that a component carrier whoseoperational frequency is that of the frequency band having the highquality should be added. In the example of FIG. 13, the base station 200decides that the operational frequency of the component carrier CC3should be changed from the third band (#3) to the fourth band (#4).

Accordingly, the base station 200 transmits a component carrier deletecommand to the user equipment 100 by specifying the component carrierCC3 (step S422). In response thereto, the user equipment 100 deletes thecomponent carrier CC3 from the component carriers in use (step S424).Then, the base station 200 transmits a component carrier add command tothe user equipment 100 by specifying the fourth band (#4) (step S426).In response thereto, the user equipment 100 acquires synchronizationwith a downlink channel of the fourth band (#4) in order to add the newcomponent carrier CC3 whose operational frequency is that of the fourthband (#4) (step S428). Note that because additional timing adjustment isnot necessary at the time of changing or adding a component carrier inthe cell of the same base station 200, it is not necessary to performrandom access unlike the case of a handover.

Through such a procedure, the user equipment 100 continues a radiocommunication with the base station 200 by using the component carriersCC1 to CC3 whose operational frequencies are those of the first, secondand fourth bands, respectively (step S430).

Note that in the case where an addition of a component carrier, ratherthan a change of a component carrier, is decided, the steps S422 andS424 shown in FIG. 13, for example, can be omitted. Then, in the stepS426, for example, an add command of a component carrier whoseoperational frequency is that of fourth or fifth band is transmittedfrom the base station 200 to the user equipment 100.

Further, the case where a component carrier is added is when a componentcarrier not in use (or an RF circuit or the like not in use) remains inthe user equipment 100. In such a case, the user equipment 100 mayperform measurement by using the component carrier not in use withoutreceiving an allocation of measurement gaps. However, in the case wherea component carrier should be added urgently, measurement can beperformed at higher speed by using the component carrier in use and thecomponent carrier not in use in parallel. Further, measurement can beperformed by using the component carrier in use also in the case whereit is not desirable to activate an RF circuit in the sleep mode for thepurpose of power saving or the like.

6. Summary

The user equipment 100 and the base station 200 included in the radiocommunication system 1 according to an embodiment of the presentinvention are described above with reference to FIGS. 3 to 13. Accordingto the embodiment, in the base station 200, an allocation of measurementgaps is controlled by the control unit 280 with respect to eachcomponent carrier according to the urgency level of a handoverdetermined by the determination unit 270. Further, the pattern of anallocation of measurement gaps is controlled according to the qualitylevels of the respective component carriers. Then, in the user equipment100, measurement for a handover is performed by using the measurementgaps allocated by the base station 200. It is thereby possible tosuppress a decrease in throughput or a delay in handover processing dueto an increase in measurement gaps in a radio communication involvingthe carrier aggregation.

Further, in the embodiment, the pattern of an allocation of measurementgaps can be controlled with respect to each component carrier accordingto the quality levels of the respective component carriers not only atthe time of a handover but also at the time of a change or an additionof a component carrier. It is thereby possible to suppress a decrease inthroughput and a delay in processing associated with a change or anaddition of a component carrier.

Although preferred embodiments of the present invention are described indetail above with reference to the appended drawings, the presentinvention is not limited thereto. It should be understood by thoseskilled in the art that various modifications, combinations,sub-combinations and alterations may occur depending on designrequirements and other factors insofar as they are within the scope ofthe appended claims or the equivalents thereof.

REFERENCE SIGNS LIST

-   1 RADIO COMMUNICATION SYSTEM-   100 USER EQUIPMENT-   110 RADIO COMMUNICATION UNIT-   160 CONTROL UNIT-   170 MEASUREMENT UNIT-   200 BASE STATION-   210 RADIO COMMUNICATION UNIT-   270 DETERMINATION UNIT-   280 CONTROL UNIT

The invention claimed is:
 1. A base station comprising: a radiocommunication unit implemented by circuitry and configured to establishcommunication with a mobile communication terminal using a plurality ofcomponent carriers; a determination unit implemented by the circuitryand configured to determine a handover factor; and a control unitimplemented by the circuitry and configured to allocate to the mobilecommunication terminal a measurement time interval for at least onecomponent carrier from the plurality of component carriers according tothe handover factor, wherein at least one component carrier from theplurality of component carriers is not allocated the measurement timeinterval based upon an urgency level indicated by the handover factor,wherein the urgency level depends on a moving speed and a communicationquality of the mobile communication terminal, and wherein the at leastone component carrier from the plurality of component carriers is notallocated the measurement time interval upon determination that themoving speed of the mobile communication terminal is below a speedthreshold and the communication quality is above a communicationthreshold.
 2. The base station according to claim 1, wherein onecomponent carrier from the plurality of component carriers has a highercommunication quality compared to each other component carrier from theplurality of component carriers, and the control unit is furtherconfigured to avoid assignment of the measurement time interval to theone component carrier.
 3. The base station according to claim 1, whereinthe control unit is further configured to adjust a frequency ofallocation of the measurement time interval to the at least onecomponent carrier according to a communication quality of the at leastone component carrier.
 4. The base station according to claim 3, whereinthe control unit is further configured to increase the frequency ofallocation of the measurement time interval in case where thecommunication quality of the at least one component carrier degrades. 5.The base station according to claim 3, wherein the control unit isfurther configured to decrease the frequency of allocation of themeasurement time interval in case where the communication quality of theat least one component carrier improves.
 6. The base station accordingto claim 1, wherein the control unit is further configured to, accordingto at least one of a communication quality and a measurement report,delete a component carrier from the plurality of component carriers usedfor communication terminal or add a component carrier from the pluralityof component carriers used for communication with the mobilecommunication terminal.
 7. A mobile communication terminal comprising: aradio communication unit implemented by circuitry and configured toestablish a communication with a base station using a plurality ofcomponent carriers; and a control unit implemented by the circuitry andconfigured to receive a measurement time interval for at least onecomponent carrier from a base station according to a handover factor,wherein at least one component carrier from the plurality of componentcarriers is not allocated the measurement time interval based upon anurgency level indicated by the handover factor, the urgency leveldepending on a moving speed and a communication quality of the mobilecommunication terminal, and wherein the at least one component carrierfrom the plurality of component carriers is not allocated themeasurement time interval upon determination that the moving speed ofthe mobile communication terminal is below a speed threshold and thecommunication quality is above a communication threshold.
 8. The mobilecommunication terminal according to claim 7, wherein one componentcarrier from the plurality of component carriers has a highercommunication quality compared to each other component carrier from theplurality of component carriers, and the one component carrier is notassigned the measurement time interval.
 9. The mobile communicationterminal according to claim 7, wherein a frequency of allocation of themeasurement time interval to the at least one component carrier isadjusted according to a communication quality of the at least onecomponent carrier.
 10. The mobile communication terminal according toclaim 9, the frequency of allocation of the measurement time interval isincreased in case where the communication quality of the at least onecomponent carrier degrades.
 11. The mobile communication terminalaccording to claim 9, wherein the frequency of allocation of themeasurement time interval is decreased in case where the communicationquality of the at least one component carrier improves.
 12. The mobilecommunication terminal according to claim 7, wherein the control unit isfurther configured to, according to at least one of a communicationquality and a measurement report, delete a component carrier from theplurality of component carriers used for the mobile communicationterminal or add a component carrier from the plurality of componentcarriers used for communication with the base station.
 13. A methodcomprising: establishing communication, by a basestation, with a mobilecommunication terminal using a plurality of component carriers;determining, by the basestation, a handover factor; and allocating, bythe basestation, to the mobile communication terminal a measurement timeinterval for at least one component carrier from the plurality ofcomponent carriers according to the handover factor, wherein at leastone component carrier from the plurality of component carriers is notallocated the measurement time interval based upon an urgency levelindicated by the handover factor, wherein the urgency level depends on amoving speed and a communication quality of the mobile communicationterminal, and wherein the at least one component carrier from theplurality of component carriers is not allocated the measurement timeinterval upon determination that the moving speed of the mobilecommunication terminal is below a speed threshold and the communicationquality is above a communication threshold.